Modern automatic transmissions for motor vehicles for implementing a plurality of gear steps typically are comprised of several planetary gear sets, which can be designed as simple planetary gear sets and/or as combined planetary gear sets, for instance Simpson gear sets, or Ravigneaux gear sets. Some of the components of these planetary gear sets, such as the sun gears, ring gears and carriers (planet carriers), are coupled together in a rotationally fixed manner, and others are selectively coupled via assigned friction shift elements, which are preferably designed as multi-disk clutches and multi-disk brakes, that is, for shifting a specific gear step, they can be coupled together to an input shaft, an output shaft or together, or they can be fixed with respect to the transmission housing.
To satisfy the requirement to permanently operate the drive motor to the greatest extent possible in a rotational speed range with low specific fuel consumption, increasing numbers of gear steps are necessary which, however, is inevitably associated with an increased number of the planetary gear sets in an automatic transmission. However, with the customary axially adjacent arrangement of the planetary gear sets, this leads to an increased axial construction length of the automatic transmissions, thereby complicating or even preventing a front transverse mounting of the drive unit which is comprised of the drive motor and the automatic transmission.
As a result, numerous concepts have been proposed for multi-stage automatic transmissions having at least two simple planetary gear sets, where one of these planetary gear sets is disposed coaxially or radially within the sun gear of the other planetary gear set. Here, both planetary gear sets can be disposed in a common radial plane or in radial planes that are slightly offset axially from each other, so that a gear set plane can be eliminated compared to an axially adjacent arrangement, thereby reducing the axial construction length of the respective automatic transmission.
The document U.S. Pat. No. 5,429,557 A discloses a first automatic transmission having a radially nested arrangement of two simple planetary gear sets. The two planetary gear sets are disposed coaxially above the output shaft, and form the main gear set of this disclosed automatic transmission. The ring gear of the radially inner planetary gear set and the sun gear of the radially outer gear set are formed integrally, that is, combined in a common gear component. The carriers of the planetary gear sets are rigidly connected together, and are in drive connection with the output shaft of the transmission.
In a first embodiment of this disclosed automatic transmission, there are six forward gears and one reverse gear that can be shifted using three clutches and two brakes, where an input side drive connection is created to the input shaft which is disposed axis parallel to the output shaft, in each case via a first input step with an underdrive transmission ratio and/or via a second input step with an overdrive transmission ratio. In a second embodiment of this automatic transmission, there are six forward gears and one reverse gear that can be shifted using two clutches and three brakes, where the input side drive connection to the input shaft is created in each case via a single input step with an underdrive transmission ratio, and as required, via a simple planetary gear set on the input side of the main gear set.
The document DE 10 2005 014 592 A1 describes a second automatic transmission with a radially nested arrangement of two simple planetary gear sets in several embodiments. This known automatic transmission comprises four simple planetary gear sets coupled together. Two of these planetary gear sets are disposed in a common radial plane, where the one planetary gear set is disposed radially within the sun gear of the other planetary gear set of the input side gear set. The ring gear of the radially inner planetary gear set and the sun gear of the radially outer gear set are preferably rigidly coupled together by means of a one-piece construction. This known automatic transmission comprises eight forward gears and one reverse gear that can be shifted using three clutches and two brakes.
A further automatic transmission with a radially nested arrangement of two simple planetary gear sets is the subject matter of the undisclosed document DE 10 2008 000 429.4 (published as DE 10 2008 000 429) from the applicant. The automatic transmission described in that document in several embodiments, comprises an input side gear set and a main gear set. Whereas the input side gear set in all embodiments comprises two coupled simple planetary gear sets, the main gear set comprises either two coupled simple planetary gear sets or a Ravigneaux gear set.
In some embodiments, the two planetary gear sets of the input side gear set are disposed in radial planes slightly offset axially from each other, and in other embodiments they are in a common radial plane, where the first planetary gear set is disposed radially within the sun gear of the second planetary gear set. The ring gear of the radially inner planetary gear set and the sun gear of the radially outer planetary gear set are rigidly connected together.
The carriers of the two planetary gear sets are likewise rigidly connected together, and depending on the embodiment, coupled in a rotationally fixed manner to the ring gear of the first planetary gear set of the main gear set, to the sun gear of the first planetary gear set of the main gear set, to the small sun gear of the Ravigneaux gear set or to the large sun gear of the Ravigneaux gear set. In each of the embodiments of the automatic transmission, there are nine forward gears and one reverse gear that can be shifted via three clutches and three brakes, where for avoiding drag moments of disengaged friction shift elements, one clutch and one brake are each designed as dog clutches.
In the described radially nested arrangement of two simple planetary gear sets, the centering of the components is problematic due to the radial expansion of the components, the high rotational speeds occurring to some extent, and the high masses of some component parts. Thus, in the presently considered coaxially nested arrangement, the ratio of the reference circle diameter of the outer ring gear, that is, the ring gear of the radially outer planetary gear set, and of the inner sun gear, that is, the sun gear of the radially inner planetary gear set, is greater than 3.5.
In addition, such an arrangement of two planetary gear sets as an input side gear set of an automatic transmission according to the undisclosed document DE 10 2008 000 429.4 (published as DE 10 2008 000 429), the shifted fourth forward gear for example, results in a superelevation of rotational speed of the coupled carriers of the two planetary gear sets of approximately 1.38, of the outer ring gear of approximately 1.63, and of the inner sun gear of approximately 2.13, relative to the input shaft in drive connection with the drive motor.
Due to the relatively large circumferential expansion of the radially outer components, particularly of the coupled carrier of the two planetary gear sets and of the outer ring gear, these components additionally have inevitably relatively high masses.
For centering a single simple planetary gear set, it is generally known to be sufficient to radially support a single component, for instance the sun gear, a ring gear carrier rigidly connected to the ring gear, or the carrier,
For example, on a centrally disposed transmission shaft with minimal radial play; however, with the currently considered radially nested arrangement of two simple planetary gear sets, this is not possible in this configuration due to the named reasons. Thus, production related eccentricities or asymmetries of components, in the case of heavy or fast rotating components, can lead to uneven rotation, increased wear particularly of the gearing and to increased operating noise. Likewise, due to their weight, the heavy components, particularly the coupled carriers and the outer ring gear, can slip radially downward when they are not rotating to the extent that self-centering of these components during the next startup of rotation is only possible with considerable delay.
FIG. 1 from the already named document U.S. Pat. No. 5,429,557 A shows a known mounting arrangement of two radially nested simple planetary gear sets, for example. In the sectional view of the first embodiment of the particular automatic transmission shown there, the sun gear of the radially inner planetary gear set is rigidly connected to an intermediate shaft designed as a hollow shaft, and supported together with it by means of a needle bearing on the centrally disposed output shaft.
The two rigidly connected carriers are supported on the axially opposite side of the two planetary gear sets using a bearing hub of the carrier of the radially inner planetary gear set using at least one needle bearing on the output shaft. The one-piece gear component in which the ring gear of the radially inner planetary gear set and the sun gear of the radially outer planetary gear sets are combined is rigidly connected to the inner disk carrier of an axially adjacent clutch, and supported via this carrier by means of a plain bearing sleeve on a bearing hub of the output gear of the first input stage. The output gear of the first input stage is in turn, supported via two needle bearings on the sun gear of the radially inner planetary gear set and the intermediate shaft connected thereto.
The ring gear of the radially outer planetary gear set is rigidly connected to the output gear of the second input stage, which in turn is supported via two needle bearings in the bearing hub of the carrier of the radially inner planetary gear set.
All components of these two planetary gear sets are forcibly guided with respect to their gearing due to the radial support. In addition, unfavorably long tolerance chains result for some components due to the multiple staggering of the radial supports. Even in the case of only a small eccentricity or ovality of a component in one of the two planetary gear sets, radial warping and uneven running inevitably occur, in conjunction with increased wear of the radial support and gearing, as well as increased operating noise.